For bobbing it doesn't matter. It never passes above the baseline. However, add some extra image below if you'd like some room to play with offsets a bit.
If the sprite meets the top of the screen, make sure it's got a few pizels above.
To be widescreen friendly, give each side an extra 16 pixels for the screen space, and another 16 to account for bobbing.
384x200 is the minimum space. Expand in total to about 400x240 for lots of room to throw the image around in.
Once you're lined up to the center point in the screen preview, in whatever wad editor you use, make sure to bump it upward by another 32 pixels. (The bottom of the screen in a full screen mode will be at the "Status bar" line. Yes it's stupid, but blame Id.)